The present invention relates to the field of oil and gas exploration, and in particular downhole activities, such as, for example, drilling, logging, fishing, completions, etc.
More specifically, the present invention relates to a tool for selectively connecting or disconnecting components of a downhole workstring, comprising a drive shaft and a housing couplable to said drive shaft, said housing comprising a first housing part and a second housing part releasably connected to one another. In particular the invention is a so-called back off sub for release and re-connection of components of a downhole workstring.
Safety subs, also referred to in the industry as back-off safety subs (BOSS), are used in downhole activities to disengage or connect components of downhole string whenever it becomes necessary. This tool is used in various workstrings such as drill, fishing, washover string. During normal downhole activities, the tool transmits torque from the upper string to the equipment below the tool. Disconnection may for example be required to recover the part of the string above the equipment which has been or stuck or otherwise fixed downhole.
Some known safety subs comprise upper and lower mating parts couplable together by way of a thread and a stop device for preventing the thread from loosening until it is required to disengage the mating parts. Such a stop device can comprise a spring or a friction ring or some other stop means which typically maintains bias between the mating parts and thus prevents the mating parts from unscrewing and maintains torsional integrity of the joint between the mating parts. When it is required to disengage the mating parts, the stop device can be acted upon from surface, for example by application of a force overcoming the bias, so as to permit the mating parts to rotate relative to each other. The upper string can then be rotated from surface while the stuck portion remains stationary thus permitting the mating parts to disengage.
Disadvantages associated with such known back off subs include, for example, the risk of accidental disengagement of the mating parts of the back off sub during the normal downhole activity and the associated financial losses due to the non-production time required to re-connect the string.
The aim of this invention is to provide a tool with the features mentioned in the first paragraph of this description, which does not present said disadvantages.
This aim is achieved by providing a tool for selectively connecting or disconnecting components of a downhole workstring comprising a drive shaft and a housing couplable to said drive shaft, said housing comprising a first housing part and a second housing part releasably connected to one another, and further comprising transmission means coupled to the drive shaft, said transmission means being arranged to selectively connect or disconnect the housing parts by rotating the drive shaft.
Preferably, said transmission means are arranged so that the rotation of said drive shaft causes a longitudinal displacement of one of the housing parts, or of an operating member for connecting or disconnecting said housing parts, and that the housing parts are connected or disconnected as a result of said axial displacement.
Advantageously, the tool comprises a kinematic pair which is selected such that said axial displacement occurs for a predetermined disconnection time before the housing parts disengage.
In a further advantageous arrangement, the tool comprises a gear reduction mechanism.